Introduction: When Constantine I was acclaimed emperor by the Roman troops in Eburacum (York) after the death of his father Constantius Chlorus in the summer of 306, this step was at once both extraordinary and predictable—and it was probably seen as such by contemporaries, whether or not they considered the Imperium Romanum a hereditary monarchy. Neither in the eyes of the new senior augustus Galerius nor in the view of most modern historians did Constantine’s accession satisfy the prevailing criteria of the time. Whether he can be called a “usurper,” however, is of secondary importance, for it is clear that he saw himself confronted by a deficit of legitimacy. His success tipped the balance in favor of the idea that being related to an emperor justified one’s claim to rule, and it was in this period that the dynastic principle was established as an explicit element of the legitimation of Roman rulers once and for all. With the exception of Jovian, who ruled for only a few months, all universally recognized emperors between 324 and the mid-fifth century, without exception, were members of only two dynasties: first the Constantinian and subsequently the Valentinian-Theodosian. he aim of this chapter is to illustrate the causes and consequences of this development.

The dynastic principle had been important in the Roman monarchy from the very beginning. he idea that property, clients, and influence—but not potestates and honores—could be inherited was self-evident to the Roman nobility of the res publica libera. It made the careers of not a few homines novi far more difficult. Theoretically, the principle of meritocracy obtained; but in reality, as in most aristocratic societies, the Roman nobility sought to limit the number of social climbers and to concentrate power in the hands of the established gentes. Without the widespread willingness of supporters and soldiers to transfer their loyalty to their patronus’s heir, Caesar’s adoptive son Octavian could never have seized power for himself in the Imperium Romanum.